1) Field of the Invention
The present invention relates to an image formation apparatus and a registration adjusting method.
2) Description of the Related Art
In an electronic photographing type image formation apparatus such as a laser printer or a digital copying machine, there has been employed a system called a tandem system. In this system, lighting of an LD (laser diode) is controlled corresponding to image data of each color component. An image holder (a photoconductor) provided for each color is main and sub scanned with each optical beam emitted from the LD. (A line scanning according to an optical beam is a main scanning, and a scanning according to a relative movement of an image holder to a direction crossing the main scanning line is a sub scanning.) An image is written based on this operation. In this case, a scanning optical system (refer to FIG. 1A, FIG. 1B and FIG. 4) is used to allocate four optical beams to left and right with a polygon mirror (a light deflector), and write an image into an image holder at a constant timing for each main scanning line scanned periodically. For this purpose, a synchronous detecting sensor that detects an optical beam at the outside of a writing area at an image write starting side is generally provided. The synchronous detecting sensor detects a passing of an optical beam scanned in the main scanning (line) direction, and starts writing an image in the main scanning direction at a predetermined timing using a generated synchronizing signal as a reference.
When an image of four color components including yellow (Y), magenta (M), cyan (C), and black (K) is written in this system, a temperature distribution around a lens of an fxcex8 lens used for an optical beam scanning optical system varies according to the operation of the apparatus. Consequently, lenses of these colors have mutually different temperatures (there are many parts that are deteriorated by dust such as lenses and mirrors inside the writing optical unit, and therefore, there is no fan to generate wind, and it is hard to keep a constant temperature). As a result, characteristics of lenses become different. Therefore, in the color simultaneous writing system as shown in FIGS. 1A and 1B, a distortion occurs in the image for each color. This is due to a distance from a synchronous detecting position of each color to an image writing starting position (hereinafter to be referred to as xe2x80x9cregistrationxe2x80x9d), or a variation in the magnification based on expansion and compression of the material of the lens.
Regarding the magnification in the above factors, a solving method has been proposed conventionally (Japanese Patent Application Laid-Open No. H10-307269). Synchronous detecting sensors disposed at two points on a scanning line detect scanning beams, and can match the beams using obtained synchronous detecting signals.
Regarding the registration, a multi-color image formation apparatus (Japanese Patent Application Laid-Open No. 2001-121739) has been proposed conventionally that uses the following method. A synchronous detecting sensor that detects a scanning optical beam of each color is provided, and this sensor measures a time interval from the occurrence of a sensor detecting signal of a color determined in advance as a reference to the occurrence of a sensor detecting signal of other color. Based on this, a relative position of a synchronous detecting sensor is obtained for each color. When a variation occurs in the position obtained as a time interval, registration is adjusted according to the variation.
According to this conventional example, however, a time interval from the occurrence of a sensor detecting signal of a color determined in advance as a reference to the occurrence of a sensor detecting signal of other color is measured, and registration is adjusted based on this variation. For this purpose, it is necessary to set the sensor of each color subject to a condition that it is always possible to measure a time interval. Actually, according to the sensors shown in the conventional example, the sensor of a color determined in advance as a reference is disposed at the upstream side in the scanning direction with a distortion relative to the sensors of other colors. In other words, the layout of the sensors is limited, and the degree of freedom of design is made small.
Further, according to the above conventional example, when a measured time interval is different from the last set value, the time interval is updated based on a value measured this time. An adjustment value is set for registration adjustment based on the updated value. Even when there is an error in the measurement, this is reflected straight to the setting. Therefore, there is a risk that errors are accumulated, and it is difficult to maintain adjustment precision.
It is an object of the present invention to provide an image formation apparatus and registration adjusting method of making it possible to increase the degree of freedom of design and improve the registration adjustment precision, by detecting a scanning optical beam of each color used for registration adjustment, without limiting the layout of a synchronous detecting unit that generates a writing reference synchronization signal.
According to one aspect of the present invention, there is provided an image formation apparatus that has a writing control unit that line scans an image holder in a predetermined period with optical beams emitted from a light source of which lighting is controlled corresponding to each image data of a plurality of colors, and forms one image by superimposing written images of respective colors. The image formation apparatus comprises a synchronous detecting unit that detects each scanning optical beam and generates a writing reference synchronization signal, and a mark pulse generating unit that generates a mark pulse at a predetermined timing from the writing reference synchronization signal. The writing control unit has a time measuring unit that measures a time from a writing reference signal generated in a color determined as a reference from among a plurality of colors to a mark pulse generated in another color, and a registration adjusting unit that adjusts a registration of each of the other colors based on a change in a measured time when this change has occurred.
According to another aspect of the present invention, there is provided a registration adjusting method in an image formation apparatus that carries out writing control in order to line scan an image holder in a predetermined period common to each with optical beams emitted from a light source of which lighting is controlled corresponding to each image data of a plurality of colors, and form one image by superimposing written images of the colors. The registration adjusting method comprises the steps of generating a writing reference synchronizing signal by detecting each scanning optical beam in an operating state, generating each mark pulse at a predetermined timing from each generated writing reference synchronizing signal, measuring a time from a writing reference signal generated in a color determined as a reference from among a plurality of colors to a mark pulse generated in other color, and adjusting a registration corresponding to a change in a measured time when this change has occurred.
These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings.